Presolar O- and C-anomalous grains in unequilibrated ordinary chondrite matrices

Abstract

Presolar grains are trace components in chondrite matrices. Their abundances and compositions have been systematically studied in carbonaceous chondrites but rarely in situ in other major chondrite classes. We have conducted a NanoSIMS isotopic search for presolar grains with O- and C-anomalous isotopic compositions in the matrices of the unequilibrated ordinary chondrites Semarkona, Meteorite Hills 00526, and Northwest Africa 8276. The matrices of even the most primitive ordinary chondrites have been aqueously altered and/or thermally metamorphosed, destroying their presolar grain populations to varying extents. In addition to randomly placed isotope maps, we targeted recently reported, relatively pristine Semarkona matrix areas to better explore the original inventory of presolar grains. In all samples, we found a total of 122 O-anomalous grains (silicates + oxides), 79 SiC grains, and 22 C-anomalous carbonaceous grains (organics, graphites). Average matrix-normalized abundances are 151 ppm O-anomalous grains, 53 ppm SiC grains and 56 ppm carbonaceous grains in Semarkona, 55 ppm (O-anom.), 22 ppm (SiC) and 3 ppm (carb.) in MET 00526 and 12 ppm (O-anom.), 15 ppm (SiC) and 1 ppm (carb.) in NWA 8276. In relatively pristine ordinary chondrites and in primitive carbonaceous and C-ungrouped chondrites, the O and C isotopic composition of presolar grains and their matrix-normalized abundances are similar, despite the likely differences in chondrite-formation time and nebular location. These results suggest a relatively homogenous distribution of presolar dust across major chondrite-forming reservoirs in the solar nebula. Secondary asteroidal processes are mainly responsible for differences in presolar grain abundances between and within chondrites, highlighting the need to identify and target the most pristine chondrite matrices for such studies.